The present invention relates to an image decoding apparatus such as a personal computer, for example, that receives digital broadcast signals where image-compressed moving pictures and program information are multiplexed, and decodes selected moving pictures to display them.
Apparatuses that capture television broadcasts and display them include an apparatus that digitizes analog television broadcasts and displays moving pictures on a personal computer.
FIG. 10 shows an example of the structure of a personal computer for receiving analog broadcasts. Reference numeral 101 represents an input high-frequency signal. Reference numeral 102 represents a tuner. Reference numeral 103 represents A/D converting means. Reference numeral 104 represents a digital video signal which is a digitized video signal. Reference numeral 10 represents graphic generating means. Reference numeral 11 represents a graphic signal. Reference numeral 12 represents overlaying means. Reference numeral 13 represents a memory. Reference numeral 14 represents memory control means. Reference numeral 22 represents a memory data signal. Reference numeral 107 represents a overlaid digital video signal. Reference numeral 16 represents D/A converting means. Reference numeral 108 represents an analog video signal. Reference numeral 18 represents display means such as a CRT display. Reference numeral 105 represents timing generating means. Reference numeral 106 represents a clock for D/A conversion.
Here, the graphic signal 11 is a signal for causing graphics such as so-called icons and window frames to be displayed on the display screen of the personal computer.
In FIG. 10, the input high-frequency signal 101 is channel-selected by the tuner 102 and is digitized by the A/D converting means 103, the digitized video signal 104 is overlaid on the graphic signal 11 by the overlaying means 12, and the overlaid digital video signal 107 is supplied to the D/A converting means 16 and is output as the analog video signal 108.
The timing generating means 105 generates a display reference clock 106, which is supplied to the memory control means 14 so as to be used as a reading reference clock for reading the overlaid digital video signal from the memory and is supplied to the D/A converting means 16 so as to be used as a D/A conversion reference clock.
However, in this conventional image display apparatus, the overlaid digital video signal 107 is read out from the memory means 13 based on the display reference clock 106 that is most suitable for display of a personal computer. For this reason, the timing of the reading and the timing of writing of the digital video signal 104 to the memory means 13 are asynchronous to each other, so that a problem arises that in the digital video signal 104, after the D/A conversion, disturbances in displayed images such as tearing artifacts of display frames and missing frames are caused on the display means 18.
This problem similarly arises when digitally compressed video signals such as MPEG-compressed video signals are captured by a personal computer in order to support digital broadcasts which will emerge in the future.
In view of this conventional problem, an object of the present invention is to provide an image decoding apparatus in which disturbances in digital video images are""substantially not caused even when digitally compressed image signals are used and overlaid on graphic signals and the overlaid image is displayed.
The first present invention(corresponding to the invention of claim 1) is an image decoding apparatus for decoding an image from a digital transport stream obtained by multiplexing a digital video bit stream being frame-coded or field-coded, and a time stamp signal, comprising:
de-multiplexing means for extracting the digital video bit stream and the time stamp signal from the digital transport stream;
first clock reproducing means for reproducing a first clock signal from the time stamp signal;
decoding means for decoding the digital video bit stream and outputting a first digital video signal having a first frame frequency and synchronizing with the first clock signal;
graphic generating means for generating a graphic signal;
overlaying means for overlaying the first digital video signal and the graphic signal and outputting a first overlaid digital signal synchronizing with the first clock signal;
memory means for storing at least the first digital video signal; and
digital-to-analog converting means for converting the first overlaid digital signal into a first analog signal in synchronization with the first clock signal.
The second present invention(corresponding to the invention of claim 2) is an image decoding apparatus for decoding an image from a digital transport stream obtained by multiplexing a digital video bit stream being frame-coded or field-coded, and a time stamp signal, comprising:
de-multiplexing means for extracting the digital video bit stream and the time stamp signal from the digital transport stream;
first clock reproducing means for reproducing a first clock signal from the time stamp signal;
second clock reproducing means for reproducing a second clock signal from the first clock signal;
decoding means for decoding the digital video bit stream and outputting a first digital video signal having a first frame frequency and synchronizing with the first clock signal;
graphic generating means for generating a graphic signal;
overlaying means for overlaying the first digital video signal and the graphic signal and outputting a second overlaid digital signal synchronizing with the second clock signal and having a second frame frequency;
memory means for storing at least the first digital video signal; and
digital-to-analog converting means for converting the second overlaid digital signal into a second analog signal in synchronization with the second clock signal.
The third present invention(corresponding to the invention of claim 3) is an image decoding apparatus for decoding an image from a digital transport stream obtained by multiplexing a digital video bit stream being frame-coded or field-coded, and a time stamp signal, comprising:
de-multiplexing means for extracting the digital video bit stream and the time stamp signal from the digital transport stream;
first clock reproducing means for reproducing a first clock signal from the time stamp signal;
second clock reproducing means for reproducing a second clock signal from the first clock signal;
decoding means for decoding the digital video bit stream and outputting a first digital video signal having a first frame frequency and synchronizing with the first clock signal;
graphic generating means for generating a graphic signal;
interpolating means for outputting a second digital video signal obtained by increasing a spatial resolution of the first digital video signal;
overlaying means for overlaying the second digital video signal and the graphic signal and outputting a third overlaid digital signal synchronizing with the second clock signal;
memory means for storing at least the second digital video signal; and
digital-to-analog converting means for converting the third overlaid digital signal into a third analog signal in synchronization with the second clock signal.
The fourth present invention(corresponding to the invention of claim 4) is an image decoding apparatus for decoding an image from a digital transport stream obtained by multiplexing a digital video bit stream being frame-coded or field-coded, and a time stamp signal, comprising:
de-multiplexing means for extracting the digital video bit stream and the time stamp signal from the digital transport stream;
first clock reproducing means for reproducing a first clock signal from the time stamp signal;
second clock reproducing means for reproducing a second clock signal from the first clock signal;
decoding means for decoding the digital video bit stream and outputting a first digital video signal having a first frame frequency and synchronizing with the first clock signal;
graphic generating means for generating a graphic signal;
decimating means for outputting a third digital video signal obtained by decreasing a spatial resolution of the first digital video signal;
overlaying means for overlaying the third digital video signal and the graphic signal and outputting a fourth overlaid digital signal synchronizing with the second clock signal;
memory means for storing at least the third digital video signal; and
digital-to-analog converting means for converting the fourth overlaid digital signal into a fourth analog signal in synchronization with the second clock signal.
The fifth present invention(corresponding to the invention of claim 5) is an image decoding apparatus according to the first present invention, further comprising:
timing generating means for generating a third clock signal independent of the first clock signal; and
switching means for choosing one clock signal between the first clock signal and the third clock signal and outputting the chosen clock signal,
wherein the digital-to-analog converting means is synchronized with the clock signal output from the switching means.
The sixth present invention(corresponding to the invention of claim 6) is an image decoding apparatus according to the second present invention, further comprising:
timing generating means for generating a third clock signal independent of the second clock signal; and
switching means for choosing one clock signal between the second clock signal and the third clock signal and outputting the chosen clock signal,
wherein the digital-to-analog converting means is synchronized with the clock signal output from the switching means.
The seventh present invention (corresponding to the invention of claim 7) is an image decoding apparatus according to the third present invention, further comprising:
timing generating means for generating a third clock signal independent of the second clock signal; and
switching means for choosing one clock signal between the second clock signal and the third clock signal and outputting the chosen clock signal,
wherein the digital-to-analog converting means is synchronized with the clock signal output from the switching means.
The eighth present invention (corresponding to the invention of claim 8) is an image decoding apparatus according to the fourth present invention, further comprising:
timing generating means for generating a third clock signal independent of the second clock signal; and
switching means for choosing one clock signal between the second clock signal and the third clock signal and outputting the chosen clock signal,
wherein the digital-to-analog converting means is synchronized with the clock signal output from the switching means.
As is apparent from the above description, the present invention has an advantage that disturbances in digital video images are substantially not caused even when a overlaid image formed by overlaying a digital video signal and a graphic signal is displayed.